In the Thermomechanical Pulping Process (TMP), wood chips are used as lignocellulosic raw matter, and their properties such as species, freshness, size, density and moisture content are important factors affecting pulp quality, as stated by Smook in “Handbook for Pulp & Paper Technologies”, Joint Textbook Committee of the Paper Industry, 54 (1982), and can have an impact on energy consumption and process stability as discussed by Garceau in “Pâtes Mécaniques et Chimico-Mécaniques. La section technique”, PAPTAC, (1989) Montreal, Canada, pp. 101 (1989). The relations between the refining process and pulp quality have been exhaustively discussed by Miles in “Refining Intensity and Pulp Quality in High-Consistency Refining”, Paperi ja Puu—Paper and Timber, 72(5): 508-514, (1990), by Stationwala et al. in “Effect of Feed Rate on Refining”, Journal of Pulp and Paper Science: vol 20 no 8 (1994) and by Wood. in “Chip Quality Effects in Mechanical Pulping—A Selected Review” 1996 Pulping Conference pp. 491-495. Furthermore, the relations between refining process and chip properties have also been exhaustively discussed by Jensen et al. in “Effect of Chip Quality on Pulp Quality and Energy Consumption in RMP Manufacture”, Int symp. on fundamental concepts of refining, Appleton Wis., Sep. (1980), by Breck et al. in “Thermomechanical Pulping—a Preliminary Optimization”, Transactions, Section technique, ACPPP, 1-3, pp 89-95 (1975) and by Eriksen et al. in “Consequences of Chip quality for Process and Pulp Quality in TMP Production”, International Conference, Mechanical Pulping, Oslo, June (1981).
According to a known control strategy, a feedback controller is used on the chip transfer screw feeder to control primary motor load, the dilution flow rate for the primary refiner being coupled with the screw feeding to operate on a constant ratio mode. Alternatively, the feedback controller can be used to control the motor load by acting upon the dilution flow rate on the basis of a pulp consistency measurement at the blow line of the primary refiner. In both cases, the variation of chip quality acts as an external disturbance affecting the motor load.
The TMP mills are large consumers of electrical energy. Disc refiners, typically powered by large 10-30 MW electric motors, are used to convert wood chips to high quality papermaking fibers. According to analysis results of M. Jackson et al. reported in “Mechanical Pulp Mill”, Energy Cost Reduction in the Pulp and Paper Industry, Browne, T. C. tech. ed., Paprican (1999), the energy consumption for a 500 BDMT/D (Bone Dry Metric Ton per Day) single-line TMP mill at 2400 kWh/BDMT, which is typical for a TMP mill using black spruce chips for newsprint production, was estimated at 2160 KWh/ADt (KWatt-hour per Air Dry ton) which corresponds to 90% of the whole mill energy consumption. Since the TMP process is used in 80% of the newsprint production worldwide, energy consumption is a major issue in that industry.
Presently, variations in specific energy consumption (SEC), i.e. applied energy per unit of weight of wood chips on an oven-dry basis during refining, to obtain a desired pulp quality can be relatively high. Usually there is a range of desired quality values, such as provided by Canadian Standard Freeness (CSF) for example, with which the produced pulp must comply to satisfy customers' demand. In this range, the obtained CSF can sometimes be near the upper limit or the lower limit. When the value is near the lower limit of the desired range, this means that more energy is needed to reach the desired quality. When the value is near the upper limit, a minimal consumption of energy for an acceptable quality pulp is reached. For cost reduction and resource protection purposes, it is desirable that energy spent to produce a pulp of a desired quality is managed efficiently.
Refiners are also involved in the manufacturing of fibreboards made from various lignocellulosic granular matters including wood chips and mill waste matters such as wood shavings, sawdust or processed wood flakes (e.g. OSB flakes). While the respective post-refining steps of fiberboard manufacturing and pulp and paper processes are distinct, their refining modes of operation are similar, and cost reduction as well as resource protection are important issues for both processes, so that it is still desirable that energy spent to produce a pulp of a desired quality is managed efficiently.